Re: [PATCH] KVM: x86: inhibit APICv when KVM_GUESTDBG_BLOCKIRQ active

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2021-11-03 at 11:29 +0200, Maxim Levitsky wrote:
> KVM_GUESTDBG_BLOCKIRQ relies on interrupts being injected using
> standard kvm's inject_pending_event, and not via APICv/AVIC.
> 
> Since this is a debug feature, just inhibit it while it
> is in use.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 1 +
>  arch/x86/kvm/svm/avic.c         | 3 ++-
>  arch/x86/kvm/vmx/vmx.c          | 3 ++-
>  arch/x86/kvm/x86.c              | 3 +++
>  4 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 88fce6ab4bbd7..8f6e15b95a4d8 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1034,6 +1034,7 @@ struct kvm_x86_msr_filter {
>  #define APICV_INHIBIT_REASON_IRQWIN     3
>  #define APICV_INHIBIT_REASON_PIT_REINJ  4
>  #define APICV_INHIBIT_REASON_X2APIC	5
> +#define APICV_INHIBIT_REASON_BLOCKIRQ	6
>  
>  struct kvm_arch {
>  	unsigned long n_used_mmu_pages;
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 8052d92069e01..affc0ea98d302 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -904,7 +904,8 @@ bool svm_check_apicv_inhibit_reasons(ulong bit)
>  			  BIT(APICV_INHIBIT_REASON_NESTED) |
>  			  BIT(APICV_INHIBIT_REASON_IRQWIN) |
>  			  BIT(APICV_INHIBIT_REASON_PIT_REINJ) |
> -			  BIT(APICV_INHIBIT_REASON_X2APIC);
> +			  BIT(APICV_INHIBIT_REASON_X2APIC) |
> +			  BIT(APICV_INHIBIT_REASON_BLOCKIRQ);
>  
>  	return supported & BIT(bit);
>  }
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 71f54d85f104c..e4fc9ff7cd944 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7565,7 +7565,8 @@ static void hardware_unsetup(void)
>  static bool vmx_check_apicv_inhibit_reasons(ulong bit)
>  {
>  	ulong supported = BIT(APICV_INHIBIT_REASON_DISABLE) |
> -			  BIT(APICV_INHIBIT_REASON_HYPERV);
> +			  BIT(APICV_INHIBIT_REASON_HYPERV) |
> +			  BIT(APICV_INHIBIT_REASON_BLOCKIRQ);
>  
>  	return supported & BIT(bit);
>  }
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ac83d873d65b0..eb3b8d2375713 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10747,6 +10747,9 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
>  	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
>  		vcpu->arch.singlestep_rip = kvm_get_linear_rip(vcpu);
>  
> +	kvm_request_apicv_update(vcpu->kvm,
> +			!(vcpu->guest_debug & KVM_GUESTDBG_BLOCKIRQ),
> +			APICV_INHIBIT_REASON_BLOCKIRQ);
I need more coffee, bad indention here :)

Could you test if this patch works for you?

I managed to reproduce this issue on AVIC, by patching out the x2apic inhibit,
and then this patch fixed the issue for me.


Best regards,
	Maxim Levitsky

>  	/*
>  	 * Trigger an rflags update that will inject or remove the trace
>  	 * flags.